1. Filed of the Invention
The present invention relates to a battery charger and a battery charging method for charging a battery and particularly relates to a battery charger and a battery charging method suited for charging a battery, such as a nickel metal hydride battery, which emits high heat while being charged.
2. Discussion of the Prior Art
Presently, a chargeable battery which can be repeatedly used for the power supply of, for example, a power tool is used.
A nickel cadmium battery is popular as a battery for the power tool, and a battery charger for quickly charging the battery by applying high current thereto is used. Specifically, the battery is quickly charged in about 20 minutes and a power tool can be continuously used by switching a battery to that which has been charged.
The inventor of the present invention studied improving the performance of a power tool by using a nickel metal hydride battery as a battery therefor. Although the nickel metal hydride battery can increase a capacity compared to a nickel cadmium battery, it generates high heat while being charged. If the temperature of the battery becomes high by the generated heat, the electrodes and separators of the cells within the battery deteriorate and battery life is shortened. Due to this, it is impossible to quickly charge the nickel metal hydride battery with high current as done for the nickel cadmium battery stated above.